TUESDAY, June 17 (HealthDay News) — When athletes think they are taking a performance-enhancing drug, their performance tends to get better — even if they never really take the drug.

So concludes a study of recreational athletes, half of whom received human growth hormone supplements while the other half took a placebo.

"This is a very relevant finding of the biology of the mind," said study co-author Dr. Ken Ho, head of the pituitary research unit at the Garvan Institute of Medical Research in Sydney, Australia. "There is a very real placebo effect at play in a sporting context, in which a favorable outcome can be achieved purely on the basis of a belief that one has received something beneficial — even if one hasn't."

Ho and his colleagues were expected to present their findings Tuesday at the Endocrine Society's annual meeting, in San Francisco.

Human growth hormone (HGH) is produced naturally by the anterior pituitary gland at the base of the brain. It is a key player in the regulation of muscle, skeletal, and organ growth. The hormone also helps process calcium and protein and stimulates the immune system.

As an injectable supplement for the purposes of boosting athletic performance, the use of HGH has been on the rise in recent years. But the World Anti-Doping Agency (WADA) notes that its use has also been linked to an increased risk for heart disease, diabetes, muscle, joint, and bone pain, high blood pressure, and osteoarthritis. WADA has therefore classified HGH as a banned substance both in and out of sports competitions.

The drug made headlines early this year when baseball great Roger Clemens denied using HGH in testimony presented at special Congressional hearings on doping in professional baseball. His former New York Yankees teammate, pitcher Andy Pettitte, has admitted to taking the drug.

Since 2004 a blood test has been in place to screen out those athletes engaged in surreptitious use. At the endocrine meeting, a separate team of researchers from Ohio University and the Aarhus Kommunehospital in Denmark presented evidence — derived from a mouse study — that points the way toward a new group of more easily identified biomarkers for HGH, which, theoretically, could lead to improved HGH screening down the road.

But Ho pointed out that "there is actually no firm scientific proof that growth hormone actually does enhance athletic performance, despite a widespread belief in its ability to do so". In fact, a review of the literature on the subject, published in March in the Annals of Internal Medicine, found no evidence that HGH could boost athletic prowess.

Ho and his team wanted to explore whether the physical boost athletes attribute to HGH might be more psychological in nature.

To do so, they focused on 64 healthy recreational athletes, men and women between the ages of 20 and 40, who had been exercising at least two hours per week over the six months prior to the study.

After testing the participants for their athletic ability, the men and women were randomized into two groups. One group got growth hormone for eight weeks, and the second received a dummy substance, or placebo. Neither the researchers nor the athletes knew which group participants were in.

At the end of the two-month trial, all the participants were asked to guess whether they had been taking HGH or a placebo, and whether their sporting performance had changed during the study period. Athletic ability was then re-tested on a range of performance parameters.

Ho and his team found that about half of the participants who received a placebo incorrectly assumed they had been given HGH. Gender played a significant role in such perceptions: the male placebo athletes were much more likely than the female athletes to have mistakenly thought they were in the HGH group.

However, regardless of gender, athletes on placebos who thought they had taken HGH typically believed their performance had improved during the study.

What's more, these "incorrect guessers" actually did improve, albeit minimally, in all measures of performance, including endurance, strength, power, and sprint capacity. In one category — high-jumping ability — the improvement was significant.

People in the placebo group who correctly guessed that they had taken a placebo improved their performance by about 1 percent to 2 percent, Ho said. But those who mistakenly thought they had taken HGH showed twice that level of overall improvement — about 2 percent to 4 percent.

"This proof of the placebo effect would equally apply to any drug, at any event, in any sport, and for any athlete, given whatever their coach is giving them," suggested Ho. "And, of course, it also goes beyond sport. It extends to health in general, and medical treatment in general."

How does this placebo effect stack up against improvements linked to actually taking HGH? Ho said his team is working on that comparison, with data coming at a later date.

Meanwhile, Dr. Michael O'Brien, an attending physician in the division of sports medicine at Children's Hospital Boston, called the finding "intriguing."

"This is one of the more unique sports supplement studies I've heard about," he noted. "Professional and elite athletes have always known that there's a very large psychological component to sports, especially with respect to endurance and recovery from hard training. But this is more evidence that more and more chemicals aren't the answer. Particularly for athletes who have a really balanced psychological approach to training."